Method and apparatus of a semiconductor-based gain equalization device for optical amplifiers

ABSTRACT

A semiconductor-based gain equalization device, method and apparatus. In one aspect of the present invention, an apparatus according to an embodiment of the present invention includes a semiconductor material. An optical path is included through the semiconductor material and is optically coupled to receive and transmit an optical beam. The gain equalization device is disposed in the semiconductor material. The optical gain equalization device includes a plurality of Bragg gratings disposed in the semiconductor material optically coupled to receive and transmit the optical beam. Each of the plurality of Bragg gratings have a different Bragg wavelength. The optical beam is to be directed from plurality of Bragg gratings with a non-uniform spectral response to compensate for the spectral non-uniformity of optical amplifiers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to optical devices and, morespecifically, the present invention relates to gain equalizationfilters.

2. Background Information

The need for fast and efficient optical-based technologies is increasingas the growth rate of Internet data traffic overtakes that of voicetraffic, pushing the need for fiber optic communications. Transmissionof multiple optical channels over the same fiber in a densewavelength-division multiplexing (DWDM) system provides a simple way touse the unprecedented capacity (signal bandwidth) offered by fiberoptics. Commonly used optical components in the system includewavelength-division multiplexing (WDM) transmitters and receivers,optical add/drop multiplexers, optical filters such as diffractiongratings, thin-film filters, fiber Bragg gratings, arrayed-waveguidegratings and optical amplifiers such as for example erbium-doped fiberamplifiers (EDFAs).

Optical amplifiers such as EDFAs, which typically operate in the C or Lwavelength band, are used to amplify optical signals. Applications forEDFAs include amplifying optical beams over for example long distancesin optical communications systems. It is well known that the opticalgain of an EDFA exhibits strong wavelength dependence. For instance, aknown EDFA has a non-uniform spectral response or a non-flat gainspectrum with gain peaks at approximately 1530 due to amplifiedspontaneous emission and 1560 nanometers. The non-uniform spectralresponse of optical amplifiers such as EDFAs presents complexities inoptical applications such as transparent DWDM lightwave networks, wheremultiple channels over a spectrum of wavelengths are included in opticalbeams. Consequently, different channels in the DWDM lightwave networksare amplified with different optical gain. Another challenge associatedwith known EDFAs is that the non-uniform spectral response of the outputis varied as a function of the optical power of the input optical signalto the EDFA. The problems associated with the non-uniform spectralresponse of EDFAs are further exacerbated when multiple EDFAs arecascaded. Known solutions to equalize the gain of EDFAs are complicatedand typically utilize complex multiple-cavity bandpass Fabry-Perot (FP)filters over the entire C-band (e.g. 1530 to 1565 nanometers).

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the accompanying figures.

FIG. 1A is a block diagram illustrating an embodiment of an opticalcommunication system including an optical amplifier having a firstnon-uniform spectral response and an optical gain equalization devicehaving a second non-uniform spectral response in accordance with theteachings of the present invention.

FIG. 1B is a block diagram illustrating another embodiment of an opticalcommunication system including an optical gain equalization devicehaving a second non-uniform spectral response and an optical amplifierhaving a first non-uniform spectral response in accordance with theteachings of the present invention.

FIG. 2 is a diagram illustrating one embodiment of a first non-uniformspectral response of an optical amplifier, a second non-uniform spectralresponse of an optical gain equalization device and a resultingsubstantially uniform spectral in accordance with the teachings of thepresent invention.

FIG. 3 is a diagram illustrating one embodiment of a gain equalizationdevice in accordance with the teachings of the present invention.

FIG. 4 is a diagram illustrating another embodiment of a gainequalization device in accordance with the teachings of the presentinvention.

FIG. 5 is a diagram illustrating a cross section of one embodiment of aBragg grating disposed in a waveguide in a semiconductor materialutilized in an optical filter of a gain equalization device inaccordance with the teachings of the present invention.

FIG. 6 is a diagram illustrating one embodiment of the peak-to-peakindex modulation and the average index for an apodized Bragg grating inaccordance with the teachings of the present invention.

FIG. 7 is a perspective diagram illustrating an embodiment of a Bragggrating disposed in a semiconductor material including a rib waveguidein accordance with the teachings of the present invention.

FIG. 8 is a diagram illustrating a cross section of another embodimentof a Bragg grating disposed in a waveguide in a semiconductor materialincluding a plurality of heaters utilized in an optical filter of a gainequalization device in accordance with the teachings of the presentinvention.

FIG. 9 is a diagram illustrating a cross section of another embodimentof a Bragg grating disposed in a waveguide in a semiconductor materialincluding a plurality of charge modulated regions utilized in an opticalfilter of a gain equalization device in accordance with the teachings ofthe present invention.

DETAILED DESCRIPTION

Methods and apparatuses for a semiconductor-based gain equalizationdevice for optical amplifiers are disclosed. In the followingdescription numerous specific details are set forth in order to providea thorough understanding of the present invention. It will be apparent,however, to one having ordinary skill in the art that the specificdetail need not be employed to practice the present invention. In otherinstances, well-known materials or methods have not been described indetail in order to avoid obscuring the present invention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthis specification are not necessarily all referring to the sameembodiment. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner in one or moreembodiments.

As an overview of the various embodiments of the present invention,semiconductor-based gain equalization devices are provided for opticalamplifiers. In one embodiment, the gain equalization device includes aplurality of Bragg gratings included in an optical filter to providefully integrated solutions on a single integrated circuit chip orsemiconductor-based optical platform. In one embodiment, each of theplurality of Bragg gratings have a different Bragg wavelength and aredesigned to provide a complex non-uniform spectral response to equalizea complex non-uniform spectral response of an optical amplifier. As willbe discussed herein, an embodiment of the gain equalization deviceincludes tunable waveguide Bragg gratings. Accordingly, tunable opticalfilters having adjustable non-uniform spectral responses are provided toadjust for variations in the non-uniform spectral responses of theoutputs of optical amplifiers. In one embodiment, a Bragg condition ofthe tunable waveguide Bragg gratings is tuned by adjusting an effectiverefractive index along the Bragg grating without making adjustments tothe grating pitch of the Bragg grating. In various embodiments, theBragg gratings are provided in the form, for example, of sampledgratings. Sampled gratings are fabricated by creating a periodicmodulation of the refractive index of the grating, generating amultiplicity of resonances.

FIG. 1A is a block diagram illustrating generally one embodiment of anoptical communication system 101 including an optical amplifier 103 andan gain equalization device 105. As shown, optical amplifier 103 isoptically coupled to receive an optical beam and amplify the signalincluded in the optical beam. The gain spectrum of optical amplifierexhibits a dependence on wavelength, which is illustrated in FIG. 1A asa first non-uniform spectral response 107. In one embodiment, opticalamplifier 103 includes a fiber amplifier such as for example anerbium-doped fiber amplifier (EDFA). In one embodiment, the optical beamamplified by optical amplifier 103 includes substantially uniformspectral response 211.

As shown in the depicted embodiment, gain equalization device 105 isoptically coupled to optical amplifier 103. In one embodiment, an inputof gain equalization device 105 is coupled to an output of opticalamplifier 103. As will be discussed, embodiments of gain equalizationdevice 105 include a semiconductor-based optical filter including aplurality of Bragg gratings disposed in semiconductor material. Theoptical filter of gain equalization device 105 is coupled to receive theoptical beam and filter the optical beam with a second non-uniformspectral response 109 to equalize the first non-uniform spectralresponse 107. As a result of the combination of the first non-uniformspectral response 107 of optical amplifier 103 and the secondnon-uniform spectral response 109 of gain equalization device 105,optical beam is amplified with a substantially uniform spectral response111.

In another embodiment, an output of gain equalization device 105 may becoupled to an input of optical amplifier 103. An example of thisembodiment is illustrated in FIG. 1B with optical communication system151. As shown, optical communication system 151 includes gainequalization device 105, which has second non-uniform spectral response109. An output of gain equalization device 105 is coupled to an input ofoptical amplifier 103, which has first non-uniform spectral response107. Accordingly, the optical beam is output with a resultingsubstantially uniform spectral response 111.

FIG. 2 is a diagram 201 including plots illustrating generally infurther detail a relationship between a first non-uniform spectralresponse 207, a second non-uniform spectral response 209 and a resultingsubstantially uniform spectral response 211 in accordance with theteachings of the present invention. In one embodiment, first and secondnon-uniform spectral responses 207 and 209 and substantially uniformspectral response 211 of FIG. 2 may correspond to first and secondnon-uniform spectral responses 107 and 109 and substantially uniformspectral response 111 of FIGS. 1A and 1B. It is appreciated that theprecise plots illustrated in diagram 201 are for explanation purposesand that there may be variations in the plots in accordance with theteachings of the present invention.

In the depicted embodiment, first non-uniform spectral response 207corresponds to an optical amplifier that is an EDFA with a non-flat gainspectrum in the C-Band. It is appreciated that other types opticalamplifiers and/or fiber amplifiers with non-uniform spectral responsesmay be utilized in other embodiments of the present invention. As shownin the depicted embodiment, first non-uniform spectral response 207includes an amplified spontaneous emission peak at approximately 1532nanometers, a minimum at approximately 1538 nanometers and then astimulated emission peak at approximately 1557 nanometers. In addition,the first non-uniform spectral response 207 drops off rapidly atwavelengths below approximately 1527 nanometers and at wavelengths aboveapproximately 1567 nanometers.

In one embodiment, an optical filter having the complex reflectancespectrum of second non-uniform spectral response 209 will substantiallyequalize the first non-uniform spectral response 207 to result insubstantially uniform spectral response 211 over the C-band.Accordingly, second non-uniform spectral response 209 includes peaks andminimas that correspond to the minimas and peaks, respectively, of firstnon-uniform spectral response 207.

In one embodiment, the complex reflectance spectrum of secondnon-uniform spectral response 209 shown in FIG. 2 is provided bydividing the reflectance spectrum into multiple ranges, with each of theranges having a corresponding peak. In the example shown in FIG. 2,ranges 213, 215 and 217 are divided at the minimas of second non-uniformspectral response 209. Accordingly, range 213 includes wavelengths belowapproximately 1532 nanometers, range 215 includes wavelengths betweenapproximately 1532 nanometers up to approximately 1557 nanometers andrange 217 includes wavelengths greater than approximately 1557nanometers. As shown in FIG. 2, range 213 includes a peak wavelength λ1of approximately 1527 nanometers, range 215 includes a peak wavelengthλ2 of approximately 1538 nanometers and range 217 includes a peakwavelength λ3 of approximately 1567 nanometers.

FIG. 3 is a diagram illustrating generally one embodiment of a gainequalization device 301 in accordance with the teachings of the presentinvention. In one embodiment, gain equalization device 301 may be usedin place of gain equalization device 105 of FIGS. 1A and 1B. In oneembodiment, gain equalization device 301 provides a complex reflectancespectrum similar to second non-uniform spectral response 209 of FIG. 2.As illustrated in the depicted embodiment, gain equalization deviceincludes a circulator 303 optically coupled to receive an optical beamand direct the optical beam to an optical filter including a pluralityof Bragg gratings 305, 307 and 309 in accordance with the teachings ofthe present invention.

Continuing with the example described above, each of the plurality ofBragg gratings 305, 307 and 309 of the optical filter have differentBragg wavelengths, which correspond to ranges 213, 215 and 217,respectively, of second non-uniform spectral response 209 of FIG. 2.Bragg grating 305 has a Bragg wavelength of λ₁, Bragg grating 307 has aBragg wavelength of λ₂ and Bragg grating 309 has a Bragg wavelength ofλ₃. In one embodiment, Bragg gratings 305, 307 and 309 waveguide Bragggratings disposed in semiconductor material on the same semiconductordie that are concatenated along a waveguide 313 disposed in thesemiconductor material. Thus, the optical beam directed from circulator303 is directed through waveguide 313 to and through Bragg grating 305,to and through Bragg grating 307 and to through Bragg grating 309. Inone embodiment, waveguide 313 is a rib waveguide disposed in thesemiconductor material on the same semiconductor die including Bragggratings 305, 307 and 309. Portions of the optical beam directed fromcirculator 303 to Bragg gratings 305, 307 and 309 having wavelengths ofλ₁, λ₂ and λ₃ are reflected back to circulator 303 through waveguide 313with a complex reflectance spectrum according to second non-uniformspectral response 209. The reflected portions of the optical beam arethen output from circulator 303 as shown.

It is appreciated that the embodiment illustrated in FIG. 3 shows gainequalization device 301 operating in reflection mode. In anotherembodiment, it is appreciated that gain equalization device 301 mayoperate in transmission mode in accordance with the teachings of thepresent invention. In this embodiment, second non-uniform spectralresponse 209 of FIG. 2 represents the transmission of light through theplurality of Bragg gratings 305, 307 and 309 in accordance with theteachings of the present invention. The Bragg gratings 305, 307, and309, work as notch optical filters in such an embodiment. In thisembodiment, the plurality of Bragg gratings 305, 307 and 309 willtransmit the optical beam with a second non-uniform transmissionspectral response 209. In this embodiment, circulator 303 is not presentand the filtered or equalized optical beam is output from gainequalization device 301 through Bragg grating 309.

FIG. 4 is a diagram illustrating generally another embodiment of a gainequalization device 401 in accordance with the teachings of the presentinvention. In one embodiment, gain equalization device 401 may be usedin place of 105 of FIGS. 1A and 1B. In one embodiment, gain equalizationdevice 401 includes an optical filter with the complex reflectancespectrum similar to second non-uniform spectral response 209 of FIG. 2.As shown in the depicted embodiment, gain equalization device 401includes a circulator 403 optically coupled to receive an optical beamand direct the optical beam to an optical filter including an opticalelement such as an optical multiplexer/demultiplexer 411 and a pluralityof Bragg gratings 405, 407 and 409. In particular, opticalmultiplexer/demultiplexer 411 in one embodiment splits or demultiplexesthe optical beam received from circulator 403 into a plurality of beamsor a plurality of optical channels with different wavelengths that areseparately directed from optical multiplexer/demultiplexer 411 to Bragggratings 405, 407 and 409 in accordance with the teachings of thepresent invention. For example, 411 may be a silicon-based arrayedwaveguide grating (AWG).

Continuing with the example described above, each of the plurality ofBragg gratings 405, 407 and 409 correspond to ranges 213, 215 and 217,respectively, of second non-uniform spectral response 209 of FIG. 2.Bragg grating 405 has a Bragg wavelength of λ₁, Bragg grating 407 has aBragg wavelength of λ₂ and Bragg grating 409 has a Bragg wavelength ofλ₃. In one embodiment, Bragg gratings 405, 407 and 409 are waveguideBragg gratings disposed in semiconductor material on the samesemiconductor die that are separately optically coupled to opticalmultiplexer/demultiplexer 411 through separate waveguides 413, 415 and417, respectively. In one embodiment, waveguides 413, 415 and 417 are arib waveguides disposed in the semiconductor material on the samesemiconductor die including Bragg gratings 405, 407 and 409.

In operation, portions of the split optical beam directed to Bragggratings 405 having a wavelength of λ₁ are reflected according to range213 back to optical multiplexer/demultiplexer 411 through waveguide 413.Portions of the split optical beam directed to Bragg gratings 407 havinga wavelength of λ₂ are reflected according to range 215 back to opticalmultiplexer/demultiplexer 411 through waveguide 415. Portions of thesplit optical beam directed to Bragg gratings 409 having a wavelength ofλ₃ are reflected according to range 217 back to opticalmultiplexer/demultiplexer 411 through waveguide 417. The reflectedportions of the optical beams according to ranges 213, 217 and 219 arethen recombined or multiplexed into an optical beam to result in areflected optical beam with a complex reflectance spectrum according tosecond non-uniform spectral response 209. This reflected optical beam isthen directed from optical multiplexer/demultiplexer 411 back tocirculator 403 and reflected optical beam is then output from circulator403 as shown.

It is appreciated that the embodiment illustrated in FIG. 4 shows gainequalization device 401 operating in reflection mode. In anotherembodiment, it is appreciated that gain equalization device 401 mayoperate in transmission mode in accordance with the teachings of thepresent invention. In this embodiment, second non-uniform spectralresponse 209 of FIG. 2 represents the transmission of light through theplurality of Bragg gratings 405, 407 and 409 in accordance with theteachings of the present invention. In this embodiment, the plurality ofBragg gratings 405, 407 and 409 will transmit the optical beam with asecond non-uniform spectral response 209. In this embodiment, theoptical beams that are output through Bragg gratings 405, 407 and 409are recombined or multiplexed to provide the filtered or equalizedoptical beam output of gain equalization device 401. In this embodiment,circulator 403 is not present.

FIG. 5 is a diagram illustrating generally a cross section of oneembodiment of a Bragg grating 501 utilized in an optical filter of again equalization device in accordance with the teachings of the presentinvention. In one embodiment, Bragg grating 501 is a sampled grating andmaybe used in place of one of the plurality of Bragg gratings 305, 307,309, 405, 407 or 409 of FIGS. 3 or 4. In the depicted embodiment, Bragggrating 501 of FIG. 5 is silicon-polysilicon grating. It is appreciatedthat silicon and polysilicon are example materials provided forexplanation purposes and that other semiconductor materials includingIII-V semiconductor materials or the like may be utilized in accordancewith the teachings of the present invention. As shown, a plurality ofregions of polysilicon 505 are disposed in a silicon semiconductormaterial 503 such that periodic, quasi-equally spaced perturbations inan effective index of refraction n_(eff) are provided along an opticalpath 517 through semiconductor material 503.

It is noted that Bragg grating 501 has been illustrated in FIG. 5 withseven polysilicon 205 regions for explanation purposes. It isappreciated that in other embodiments, Bragg grating 501 may include agreater or fewer number of polysilicon 505 regions in accordance withthe teachings of the present invention.

In one embodiment in which silicon and polysilicon are utilized, havingeffective refractive indexes of n_(Si) and n_(poly), respectively, asmall effective refractive index difference Δn_(eff) (orn_(poly)−n_(Si)) is provided at each interface between semiconductormaterial 503 and polysilicon 505. In one embodiment, Δn_(eff) isapproximately within the range of 0.005 to 0.03. It is appreciated thatother value ranges for Δn_(eff) may be utilized in accordance with theteachings of the present invention and that 0.005 to 0.03 is providedherewith for explanation purposes.

As illustrated in FIG. 5, semiconductor material 503 is included in oneembodiment in a silicon-on-insulator (SOI) wafer 515. As such, aninsulating layer 507 or a buried oxide layer is disposed betweensemiconductor material 503 and another semiconductor material 513. Inone embodiment, an additional insulating layer 509 is included such thatsemiconductor material 503 is disposed between insulating layers 507 and509. In one embodiment, insulating layer 509 is an interlayer dielectriclayer of the SOI wafer 515. In one embodiment, insulating layers 507 and509 include an oxide material or the like. As a result, a waveguide 525including optical path 517 is provided in semiconductor material 503with cladding provided by insulating layers 507 and 509.

In one embodiment, waveguide 525 is a rib waveguide. To illustrate, FIG.6 is a perspective view illustration showing generally one embodiment ofa rib waveguide 625 of a Bragg grating in accordance with the teachingsof the present invention. In one embodiment, rib waveguide 625 isdisposed between insulating regions 507 and 509 of SOI wafer 515 of FIG.5.

Referring back to FIG. 6, rib waveguide 625 is disposed in semiconductormaterial 603 and includes regions of polysilicon 605. In one embodiment,semiconductor material 603 has a different index of refraction thanpolysilicon 605 such that periodic or quasi-periodic perturbations in aneffective index of refraction are provided along an optical path throughrib waveguide 625.

As shown, the rib waveguide 625 includes a rib region 627 and a slabregion 629. In the embodiment illustrated in FIG. 6, the intensitydistribution of a single mode optical beam 619 is shown propagatingthrough the rib waveguide 625. As shown, the intensity distribution ofoptical beam 619 is such that of the majority of the optical beam 619propagates through a portion of rib region 627 towards the interior ofthe rib waveguide 625. In addition, a portion of optical beam 619propagates through a portion of slab region 629 towards the interior ofthe rib waveguide 625. As also shown with the intensity distribution ofoptical beam 619, the intensity of the propagating optical mode of beam619 is vanishingly small at the “upper corners” of rib region 627 aswell as the “sides” of slab region 629.

Referring back to the illustration in FIG. 5, an optical beam 519 isdirected along optical path 517 into one end of waveguide 525. In oneembodiment, optical beam 519 is received from an optical amplifier suchas for example optical amplifier 103 of FIGS. 1A and 1B. In oneembodiment, optical beam 519 includes infrared or near infrared lightand is confined with cladding provided by insulating layers 507 and 509to remain within waveguide 525 along optical path 217. It is appreciatedthat silicon and polysilicon are partially transparent to infrared ornear infrared light. Optical beam 519 is confined to remain withinwaveguide 525 as a result of total internal reflection since the oxidematerial of insulating layers 507 and 509 has a smaller index ofrefraction than the semiconductor material of semiconductor material 503and polysilicon 505.

As mentioned above, there are periodic, quasi-equally spacedperturbations in an effective index of refraction along optical path 517through waveguide 525. As a result of the effective refractive indexdifference Δn_(eff) described above, a multiple reflection of opticalbeam 519 occurs at the interfaces between semiconductor substrate 503and polysilicon 505 along optical path 517. In one embodiment, a Braggreflection occurs when a Bragg condition or phase matching condition issatisfied. For uniform Bragg gratings, when the conditionmλ_(B)=2n_(eff)·Λ,  (Equation 1)is satisfied, where m is the diffraction order, λ_(B) is the Braggwavelength, n_(eff) is the effective index of the waveguide and Λ is thespatial period of the grating, a Bragg reflection occurs.

In one embodiment, the reflected portions of optical beam 519 matchingthe Bragg condition, or Bragg wavelength λ_(B) are directed back out ofwaveguide 525 according to for example the particular range 213, 215 or217 of second non-uniform spectral response 209. In addition, theremainder of optical beam 519 continues to propagate along optical path517 through waveguide 525. For an embodiment in which Bragg grating 501is operating in transmission mode as discussed above, the remainder ofoptical beam 519 that continues to propagate through waveguide 525 isfiltered according to for example the particular range 213, 215 or 217of second non-uniform spectral response 209.

In one embodiment, Bragg grating 501 is an apodized grating.Accordingly, multiple side-lobes of the central lobe of the Braggwavelength λ_(B) are reduced or eliminated from second non-uniformspectral response 209. It is appreciated that side-lobes are typicallyundesirable, particularly in embodiments in which multiple Bragggratings are concatenated. FIG. 7 is a diagram 701 illustratinggenerally one embodiment of a peak-to-peak index modulation 703 and anaverage index 705 for an apodized Bragg grating along the opticalpropagation direction (e.g., z-axis) of a Bragg grating in accordancewith the teachings of the present invention. As shown in the depictedembodiment, the envelope of the index variations of peak-to-peak indexmodulation 703 has a Gaussian shape and the average index 705 issubstantially constant along the z-axis. By keeping a substantiallyconstant average index of refraction along the z-axis of the Bragggrating, the local Bragg wavelength λ_(B) remains substantiallyunchanged along the length of the Bragg grating.

In one embodiment, index variations as illustrated with peak-to-peakindex modulation 703 may be generated, for example, by first reducingthe thickness of adjacent polysilicon trenches relative to a centertrench, and then slightly increasing the thickness of the followingpolysilicon trenches. This pattern is repeated such that the narrowestpolysilicon trenches are at either side of the grating.

FIG. 8 is a diagram illustrating generally a cross section of anotherembodiment of a Bragg grating 801 disposed in a waveguide 825 in asemiconductor material 803 including a plurality of heaters 816 inaccordance with the teachings of the present invention. In oneembodiment, Bragg grating 801 is a sampled Bragg grating and may beincluded in an optical filter of a gain equalization device inaccordance with the teachings of the present invention. In the depictedembodiment, Bragg grating 801 of FIG. 8 is a silicon-polysilicon gratinghaving substantially uniform spacing. As shown, a plurality of regionsof polysilicon 805 are disposed in a silicon semiconductor material 803such that periodic or quasi-periodic perturbations in an effective indexof refraction n_(eff) are provided along an optical path 817 throughsemiconductor material 803.

Similar to the embodiment of Bragg grating 501 illustrated in FIG. 5,semiconductor material 803 is included in one embodiment an SOI wafer815. As such, an insulating layer 807 or a buried oxide layer isdisposed between semiconductor material 803 and another semiconductormaterial 813. In one embodiment, an additional insulating layer 809 isincluded such that semiconductor material 803 is disposed betweeninsulating layers 807 and 809. As a result, a waveguide 825 includingoptical path 817 is provided in semiconductor material 803 with claddingprovided by insulating layers 807 and 809. In one embodiment, waveguide825 is a rib waveguide, similar to rib waveguide 625 illustrated in FIG.6.

As shown in the depicted embodiment, Bragg grating 801 includes aplurality of heaters 811A, 811B, 811C, 811D, 811E, 811F and 811Garranged along waveguide 825. It is noted that Bragg grating 801 hasbeen illustrated in FIG. 8 with seven heaters 811A, 811B, 811C, 811D,811E, 811F and 811G for explanation purposes. It is appreciated that inother embodiments, Bragg grating 801 may include a greater or a fewernumber of heaters in accordance with the teachings of the presentinvention.

In one embodiment, the plurality of heaters have varying dimensions suchas for example thickness, height, width, etc., or may be made ofdifferent materials to provide a temperature gradient along Bragggrating 801. In one embodiment, the plurality of heaters 811A, 811B,811C, 811D, 811E, 811F and 811G include thin-film heaters or the like orother future arising technology to control the temperature ofsemiconductor substrate 803 and polysilicon 805 in waveguide 825 alongoptical path 817.

Silicon and polysilicon have large index of refraction variations withtemperature on the order of approximately 2×10⁻⁴/° K. It is appreciatedthat the index of refraction variations with temperature forsemiconductor materials such as silicon and/or polysilicon are twoorders of magnitude greater than other materials such as for examplesilica or the like. Thus, by controlling the temperature ofsemiconductor substrate 803 and polysilicon 805, relatively significantshifts in the index of refraction along optical path 817 are provided inaccordance with the teachings of the present invention.

In one embodiment, the temperature gradient along optical path 817 isvaried to result in Bragg grating 801 being an apodized grating havingfor example index modulations as illustrated with peak-to-peak indexmodulation 603 in accordance with the teachings of the presentinvention. In one embodiment, the Bragg condition λ_(B) may be varied oradjusted by varying the temperature along the optical path 817 ofwaveguide 825 to vary the effective index of refraction n_(eff), whichvaries the “2n_(eff)” term of Equation 1. Accordingly, the Braggcondition λ_(B) is adjusted without having to adjust the spatial periodΛ of Bragg grating 801. In one embodiment, the Bragg condition λ_(B) isvaried to adjust second non-uniform spectral response 209. For instance,the first non-uniform spectral response 207 of the output of for exampleoptical amplifier 103 may be varied as a function of the optical powerof the input optical signal. Accordingly, adjustments to the secondnon-uniform spectral response 209 may be made in accordance with theteachings of the present invention by adjusting the temperature gradientalong Bragg grating 801.

In operation, an optical beam 819 is directed into waveguide 825 andreflected portions of optical beam 819 matching the Bragg condition arereflected. In one embodiment, the optical beam 819 directed intowaveguide 825 is received from an optical amplifier and the reflectedportions of optical beam 819 matching the Bragg condition are directedback out of waveguide 825 according to for example the particular range213, 215 or 217 of second non-uniform spectral response 209. Inaddition, the remainder of optical beam 819 continues to propagate alongoptical path 817 through waveguide 825.

In one embodiment, the plurality of heaters 811A, 811B, 811C, 811D,811E, 811F and 811G are responsive to a control signal V_(HEAT) 816 toadjust the temperature along the optical path 817 of waveguide 825. Inanother embodiment, each of the plurality of heaters 811A, 811B, 811C,811D, 811E, 811F and 811G have similar dimensions and a separate controlsignal V_(HEAT) 816 is applied to each respective heater. In such anembodiment, each of the separate one of the control signals V_(HEAT) 816are set to values that will result in the plurality of heaters 811A,811B, 811C, 811D, 811E, 811F and 811G providing a temperature gradientalong Bragg grating 801.

FIG. 9 is a diagram illustrating generally a cross section of anotherembodiment of a Bragg grating 901 disposed in a waveguide 925 insemiconductor material including a plurality of charge modulated regions931 in accordance with the teachings of the present invention. In oneembodiment, Bragg grating 901 is a sampled Bragg grating and may beincluded in an optical filter of a gain equalization device inaccordance with the teaching of the present invention.

As shown in the depicted embodiment, Bragg grating 901 includessemiconductor material 903 having an optical path 917 through which anoptical beam 919 is directed. In one embodiment, semiconductor material903 is included in an SOI wafer 915 such that semiconductor material 903is disposed between a buried insulating layer 907 and insulating layer909. In addition, buried insulating layer 907 is disposed betweensemiconductor material 903 and semiconductor material 913. In oneembodiment, an optical waveguide 925 is provided with semiconductormaterial 903 with insulating layers 907 and 909 serving as cladding toconfine optical beam 919 to remain within waveguide 925. In oneembodiment, waveguide 925 is a rib waveguide, similar to rib waveguide625 illustrated in FIG. 6.

In the embodiment depicted in FIG. 9, an apodized Bragg grating 901 isprovided with a plurality of insulated electrodes 911A, 911B, 911C,911D, 911E, 911F and 911G distributed along a semiconductor material903. Accordingly, a plurality of conductor-insulator-semiconductorstructures, similar to, for example, metal-oxide-semiconductor (MOS)structures, are disposed along optical path 917 in semiconductormaterial 903. It is noted that Bragg grating 901 has been illustrated inFIG. 9 with seven insulated electrodes 911A, 911B, 911C, 911D, 911E,911F and 911G for explanation purposes. It is appreciated that in otherembodiments, Bragg grating 901 may include a greater or a fewer numberof insulated electrodes in accordance with the teachings of the presentinvention.

As shown in the depicted embodiment, insulated electrodes 911A, 911B,911C, 911D, 911E, 911F and 911G are coupled to receive modulationsignals V_(G1), V_(G2), V_(G3) . . . V_(GN), respectively, throughinsulating layer 909. As shown in FIG. 9, the height of each insulatedelectrode structures in waveguide 925 is h. In one embodiment, theheight h of the structures 915 is chosen such that propagation loss ofoptical beam 917 in waveguide 925 along optical path 517 is acceptable.

In the embodiment depicted in FIG. 9, periodic or quasi-periodicperturbations in an effective index n_(eff) of refraction are providedalong an optical path 917 through waveguide 925 in semiconductormaterial 903. In the illustrated embodiment, the effective index ofrefraction n_(eff) is related or equal to a function of the geometry ofwaveguide 925 along optical path 917 as well as the index of refractionof the specific medium (e.g. n_(Si)) and the wavelength or wavelengths λincluded in optical beam 919. As shown in FIG. 9, the height ofwaveguide 925 in the portions not including the insulated electrodestructures is H. Accordingly, assuming semiconductor material 903includes silicon, the effective index of refraction n_(eff) is afunction of the height H of waveguide 925 in the portions not includingthe insulated electrode structures, n_(Si) and λ. In the regions 905 ofwaveguide 925 including the insulated electrode structures, theeffective index of refraction n′_(eff) is a function of the height (H-h)of waveguide 925, n_(Si) and λ. Thus, the difference in effective indexof refraction

 Δn_(eff=n) _(eff−n′) _(eff).  (Equation 2)

In the depicted embodiment, insulated electrodes 911A, 911B, 911C, 911D,911E, 911F and 911G are biased in response to modulation signals V_(G1),V_(G2), V_(G3) . . . V_(GN), respectively. Thus, there is an increasedconcentration of free charge carriers in charge modulated regions 931 inthe semiconductor material 903 proximate to insulated electrodestructures 911A, 911B, 911C, 911D, 911E, 911F and 911G. For example,assuming a positive voltage is applied with modulation signals V_(G1),V_(G2), V_(G3) . . . V_(GN), electrons in semiconductor material 903 areswept into charge-modulated regions 931. When for example less positivevoltage is applied to the insulated electrode structures 911A, 911B,911C, 911D, 911E, 911F and 911G, the concentration of free chargecarriers swept into charge-modulated regions 931 is reduced.

It is noted that for explanation purposes, charge modulated regions 931have been illustrated using electrons or negative charge. It isappreciated that in another embodiment, the polarities of these chargesand the voltages of modulation signals V_(G1), V_(G2), V_(G3) . . .V_(GN) may be reversed. Thus, in such an embodiment, holes or positivecharge carriers are swept into charge-modulated regions 931 inaccordance with the teachings of the present invention. In anotherembodiment, the polarities of modulation signals V_(G1), V_(G2), V_(G3). . . V_(GN) may be alternated in accordance with the teachings of thepresent invention.

In one embodiment, the effective index of refraction n_(eff) incharge-modulated regions 931 is modulated in response to modulationsignals V_(G1), V_(G2), V_(G3) . . . V_(GN) due to the plasma opticaleffect. The plasma optical effect arises due to an interaction betweenthe optical electric field vector and free charge carriers that may bepresent along the optical path 917 of the optical beam 919. The electricfield of the optical beam 919 polarizes the free charge carriers andthis effectively perturbs the local dielectric constant of the medium.This in turn leads to a perturbation of the propagation velocity of theoptical wave and hence the refractive index for the light, since therefractive index is simply the ratio of the speed of the light in vacuumto that in the medium. The free charge carriers are accelerated by theelectric field, and also lead to absorption of the optical field asoptical energy is used up. Generally the refractive index perturbationis a complex number with the real part is related to the group velocitychange and the imaginary part is related to the free charge carrierabsorption. In the case of the plasma optical effect in silicon, theeffective change in the index of refraction Δn_(eff) due to the freeelectron (ΔN_(e)) and hole (ΔN_(h)) concentration change is given by:$\begin{matrix}{{\Delta\quad n_{eff}} = {{- \frac{{\mathbb{e}}^{2}\lambda^{2}}{8\pi^{2}c^{2}ɛ_{0}n_{0}}}\left( {\frac{\Delta\quad N_{e}}{m_{e}^{*}} + \frac{\Delta\quad N_{h}}{m_{h}^{*}}} \right)}} & \left( {{Equation}\quad 3} \right)\end{matrix}$where n_(o) is the nominal index of refraction for silicon, e is theelectronic charge, c is the speed of light, ε₀ is the permittivity offree space, m_(e)* and m_(h)* are the electron and hole effectivemasses, respectively.

In one embodiment, the concentration of free charge carriers swept intoeach of the charge modulated regions 931 is responsive to the voltagesof modulation signals V_(G1), V_(G2), V_(G3) . . . V_(GN). Accordingly,the effective index of refraction n_(eff) provided along optical path917 through semiconductor material 903 is responsive to modulationsignals V_(G1), V_(G2), V_(G3) . . . V_(GN). In one embodiment, thevoltages of modulation signals V_(G1), V_(G2), V_(G3) . . . V_(GN) thatare applied across the insulated electrodes 911A, 911B, 911C, 911D,911E, 911F and 911G along waveguide 925 have a voltage gradient toprovide an apodized grating. Thus, the concentration of free chargecarriers swept into each of the charge-modulated regions 931 variesalong waveguide 925 to provide the apodized-grating characteristic ofBragg grating 901.

In one embodiment, the Bragg condition λ_(B) is varied by varying thevoltages of modulation signals V_(G1), V_(G2), V_(G3) . . . V_(GN) tovary the effective index of refraction n_(eff), which varies the“2n_(eff)” term of Equation 1. Accordingly, the Bragg condition λ_(B) isadjusted without having to adjust the spatial period Λ of Bragg grating901. In one embodiment, the Bragg condition λ_(B) is varied to adjustsecond non-uniform spectral response 209 to for example adjust forvariations in the first non-uniform spectral response 207 of the outputof for example an optical amplifier 103.

In operation, an optical beam 919 is directed into waveguide 925 andreflected portions of optical beam 919 matching the Bragg condition arereflected. In one embodiment, the optical beam 919 directed intowaveguide 925 is received from an optical amplifier and the reflectedportions of optical beam 919 matching the Bragg condition are directedback out of waveguide 925 according to for example the particular range213, 215 or 217 of second non-uniform spectral response 209. Inaddition, the remainder of optical beam 919 continues to propagate alongoptical path 917 through waveguide 925.

In one embodiment, it is appreciated that relatively low voltages areutilized for modulation signals V_(G1), V_(G2), V_(G3) . . . V_(GN). Forinstance, voltages in the range of for example 5 to 15 volts are usedfor modulation signals V_(G1), V_(G2), V_(G3) . . . V_(GN) in oneembodiment. In another embodiment, the spacing between each of theinsulated electrodes 911A, 911B, 911C, 911D, 911E, 911F and 911G alongwaveguide 925 is varied such that the spacing between each of theinsulated electrodes results in an apodized grating. In such anembodiment, each of the modulation signals V_(G1), V_(G2), V_(G3) . . .V_(GN) may have similar or uniform voltages since the apodization ofBragg grating 901 in waveguide 925 will be realized by the spacingbetween insulated electrodes 911A, 911B, 911C, 911D, 911E, 911F and911G.

In the foregoing detailed description, the method and apparatus of thepresent invention have been described with reference to specificexemplary embodiments thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

1. An apparatus, comprising: a semiconductor material; an optical paththrough the semiconductor material, the optical path optically coupledto receive an optical beam; and an optical filter disposed in thesemiconductor material, the optical path including the optical filter,the optical filter including a plurality of Bragg gratings disposed inthe semiconductor material optically coupled to receive the opticalbeam, each of the plurality of Bragg gratings having a different Braggwavelength, the optical beam to be directed from the optical filter witha non-uniform spectral response in response to the plurality of Bragggratings, wherein each of the plurality of Bragg gratings includes aplurality of perturbations of a refractive index in the semiconductormaterial along the optical path, wherein the plurality of perturbationsof the refractive index are provided with regions of silicon andpolysilicon disposed in the semiconductor material along the opticalpath of each one of the plurality of Bragg gratings.
 2. The apparatus ofclaim 1 wherein each of the plurality of Bragg gratings are concatenatedalong a waveguide disposed in the semiconductor material.
 3. Theapparatus of claim 2 further comprising a circulator optically coupledto the concatenated plurality of Bragg gratings, the circulatoroptically coupled to receive the optical beam and direct the opticalbeam to the concatenated plurality of Bragg gratings, the circulatoroptically coupled to receive the optical beam directed from theconcatenated plurality of Bragg gratings with the non-uniform spectralresponse in response to the plurality of Bragg gratings.
 4. Theapparatus of claim 2 wherein the waveguide comprises a rib waveguide. 5.The apparatus of claim 1 wherein the optical filter comprises: anoptical multiplexer/demultiplexer optically coupled to receive andtransmit the optical beam; and a plurality of waveguides disposed in thesemiconductor material optically coupled to the opticalmultiplexer/demultiplexer, each of the plurality of waveguides includinga respective one of the plurality of Bragg gratings and opticallycoupled to receive and transmit the optical beam.
 6. The apparatus ofclaim 5 further comprising a circulator optically coupled to the opticalmultiplexer/demultiplexer, the circulator optically coupled to receiveand transmit the optical beam and direct the optical beam to the opticalmultiplexer/demultiplexer, the circulator optically coupled to receiveand transmit the optical beam directed from the opticalmultiplexer/demultiplexer with the non-uniform spectral response inresponse to the plurality of Bragg gratings.
 7. The apparatus of claim 5wherein each of the plurality of waveguides comprises a respective ribwaveguide disposed in the semiconductor material.
 8. The apparatus ofclaim 1 wherein the optical beam includes said one or more opticalchannels centered in wavelength bands located within a range ofapproximately 1530 to 1565 nanometers.
 9. The apparatus of claim 1wherein each one of the plurality of Bragg gratings is an apodized Bragggrating.
 10. The apparatus of claim 1, further including a plurality ofadjustable heaters disposed proximate to each one of the plurality ofBragg gratings to adjust a temperature in the semiconductor materialalong the optical path of each one of the plurality of Bragg gratings,wherein an effective index of refraction along the optical path of eachone of the plurality of Bragg gratings is responsive to the temperaturealong the optical path of each one of the plurality of Bragg gratings.11. The apparatus of claim 10 wherein the non-uniform spectral responseof the optical filter is coupled to be adjusted in response to thetemperature in the semiconductor material along the optical path of eachone of the plurality of Bragg gratings.
 12. The apparatus of claim 1wherein the plurality of perturbations of the refractive index areprovided with a plurality of adjustable charge-modulated regionsdisposed in the semiconductor material along the optical path of eachone of the plurality of Bragg gratings, the plurality of adjustablecharge modulated regions provided with a plurality of insulatedelectrodes distributed along the optical path of each one of theplurality of Bragg gratings.
 13. The apparatus of claim 12 wherein thenon-uniform spectral response of the optical filter is coupled to beadjusted in response to plurality of adjustable charge-modulated regionsalong the optical path of each one of the plurality of Bragg gratings.14. A method, comprising: directing an optical beam through asemiconductor material to a plurality of Bragg gratings disposed in thesemiconductor material, each one of the plurality of Bragg gratingshaving a different Bragg wavelength; and directing different portions ofthe optical beam from each one of the plurality of Bragg gratings inresponse to the different Bragg wavelengths, wherein the optical beam isdirected from the semiconductor material with a non-uniform spectralresponse, the non-uniform spectral response to be combined with anon-uniform spectral response of an optical amplifier to equalize thenon-uniform spectral response of the optical amplifier.
 15. The methodof claim 14 further comprising adjusting one or more of the differentBragg wavelengths of the plurality of Bragg gratings so as to adjust thenon-uniform spectral response.
 16. The method of claim 14 wherein eachof the different portions of the optical beam directed from theplurality of Bragg gratings are reflected from a respective one of theplurality of Bragg gratings.
 17. The method of claim 14 wherein saiddirecting the optical beam through the semiconductor material to theplurality of Bragg gratings disposed in the semiconductor materialcomprises: directing the optical beam through the semiconductor materialto a first one of the plurality of Bragg gratings, the first one of theplurality of Bragg gratings having a first Bragg wavelength; anddirecting the optical beam through the first one of the plurality ofBragg gratings to a next one of the plurality of Bragg gratings, thenext one of the plurality of Bragg gratings having a second Braggwavelength.
 18. The method of claim 14 further comprising: splitting theoptical beam into a plurality of optical beams, each of the plurality ofoptical beams having a different wavelength; directing each one of theplurality of optical beams to a corresponding one of the plurality ofBragg gratings.
 19. The method of claim 18 further comprisingrecombining the plurality of optical beams having the differentwavelengths from each one of the plurality of Bragg gratings into theoptical beam directed from the semiconductor material with thenon-uniform spectral response.
 20. The method of claim 19 wherein saidsplitting the optical beam into the plurality of optical beams havingthe different wavelengths comprises demultiplexing the optical beam intothe plurality of optical channels, each one of the plurality of opticalbeams including a different range of wavelengths.
 21. The method ofclaim 20 wherein said recombining the plurality of optical beams havingthe different wavelengths from each one of the plurality of Bragggratings comprises multiplexing the different portions of the opticalbeam directed from each one of the plurality of Bragg gratings into theoptical beam directed from the semiconductor material with thenon-uniform spectral response.
 22. A method comprising: directing anoptical beam through a semiconductor material to a plurality of Bragggratings disposed in the semiconductor material, each one of theplurality of Bragg gratings having a different Bragg wavelength;directing different portions of the optical beam from each one of theplurality of Bragg gratings in response to the different Braggwavelengths, wherein the optical beam is directed from the semiconductormaterial with a non-uniform spectral response; splitting the opticalbeam into a plurality of optical beams, each of the plurality of opticalbeams having a different wavelength; directing each one of the pluralityof optical beams to a corresponding one of the plurality of Bragggratings; and recombining the plurality of optical beams having thedifferent wavelengths from each one of the plurality of Bragg gratingsinto the optical beam directed from the semiconductor material with thenon-uniform spectral response.
 23. The method of claim 22 wherein saiddirecting the optical beam through the semiconductor material to theplurality of Bragg gratings disposed in the semiconductor materialcomprises: directing the optical beam through the semiconductor materialto a first one of the plurality of Bragg gratings, the first one of theplurality of Bragg gratings having a first Bragg wavelength; anddirecting the optical beam through the first one of the plurality ofBragg gratings to a next one of the plurality of Bragg gratings, thenext one of the plurality of Bragg gratings having a second Braggwavelength.
 24. The method of claim 22 wherein each of the differentportions of the optical beam directed from the plurality of Bragggratings are reflected from a respective one of the plurality of Bragggratings.
 25. The method of claim 22 wherein said splitting the opticalbeam into the plurality of optical beams having the differentwavelengths comprises demultiplexing the optical beam into the pluralityof optical channels, each one of the plurality of optical beamsincluding a different range of wavelengths.
 26. The method of claim 25wherein said recombining the plurality of optical beams having thedifferent wavelengths from each one of the plurality of Bragg gratingscomprises multiplexing the different portions of the optical beamdirected from each one of the plurality of Bragg gratings into theoptical beam directed from the semiconductor material with thenon-uniform spectral response.
 27. The method of claim 22 furthercomprising adjusting one or more of the different Bragg wavelengths ofthe plurality of Bragg gratings so as to adjust the non-uniform spectralresponse.